Typically Used in SMT Circuit Boards
The simplest printed circuit boards consist of a conductive layer, usually made out of copper, encased in two layers of strong, insulating dielectric materials like fiberglass. The first layer is the substrate, the outer mechanical layer, and the second is called the copper clad laminate (CCL). The CCL provides structural integrity and prevents the copper from oxidizing during the assembly and soldering process. In addition, the copper clad laminate has to meet several technical requirements such as its coefficient of thermal expansion (CTE), glass transition temperature (Tg), and decomposition temperature (Td).
The copper clad laminate used in smt circuit board is typically made from a combination of copper and other materials such as polyimide, phenolic, or FR-4. The choice of these materials is based on the substrate’s mechanical and electrical properties. The most important considerations are the substrate’s trackability, dielectric constant and loss tangent, and arc resistance. High-frequency digital signals require low loss materials to minimize signal degradation due to the dielectric constant, while a high-trackability material is essential for achieving good performance at low costs.
A conductive layer is typically plated on the substrate using an additive or subtractive method. The subtractive method involves etching the copper, while the additive technique applies metal vias to the substrate using a photoresist mask. The mask is patterned in the desired pattern using a photographic printing process, and the unexposed area of the board is covered with a resist material to protect it from the etching solution. The pattern is then exposed to light through the mask, and etched into the copper.
What Materials Are Typically Used in SMT Circuit Boards?
After the etching, the PCB must be cleaned to remove any chemical residue that would interfere with future manufacturing steps. The cleaning agent should be non-flammable, nontoxic, have a low melting point, and not react with other chemicals. The cleaning liquid should also have a low surface tension to avoid the formation of bubbles on the copper surface during cleaning.
Another critical part of the SMT process is applying solder paste. This is a putty-like mixture of powdered metal solder and glue, which holds the surface mount components in place and cleans the soldering surface of impurities and oxidation. It must be easy to apply, spread evenly, and adhere firmly to the soldering surface, and it must withstand high temperatures for long periods of time.
SMT is an attractive option for electronic manufacturers because it enables the use of smaller, thinner components that are cheaper and easier to handle. However, the small lead spaces of SMDs make repairing damage or failure more difficult and may even increase the risk of short-circuiting, which can occur when the solder connections aren’t properly heat treated. The process is therefore not suitable for products that need to be repaired or replaced frequently. This is especially true of mobile devices, which have a higher failure rate and are subject to more intense environmental stresses than other products. The SMT process also requires a larger number of production machines, which increases the cost of manufacture and reduces the speed at which the product can be produced.